The invention relates to hydrophobically aftercoated pearl luster pigments and to a process for preparing them. The invention further relates to the use of these pigments.
EP 0 632 109 discloses for example the subjecting of pearl luster pigments for coating compositions to an aftertreatment in order to improve their stability. In the course of this aftertreatment the pigments are coated with a layer comprising silicon dioxide, aluminum oxide, chromium oxide and/or zirconium oxide and a layer comprising an organofunctional coupling reagent, e.g. hydrophilic silanes or zirconium aluminates.
The organofunctional silanes used in EP 0 632 109 possess the following basic structure:
Yxe2x80x94(CH2)nxe2x80x94SiX3 
in which n is 1-8, Y is an organofunctional group, for example an alkyl or substituted alkyl, an aryl or substituted aryl, a halogen, an amino, a methacrylic, a vinyl and/or an epoxy group, preferably fluorine and X is a silicon-functional group which following its hydrolysis reacts with active sites of an inorganic substrate or, by condensation, with other silicon compounds. In this case X may be, for example, an alkoxy group, a hydroxy, or a halogen.
The hydrophilic groups Y react with the polymers of a coating composition to form bonds, while the silicon-functional group X reacts with the pigment surface.
Further hydrophilic silane coupling reagents containing reactive Y groups for the aftercoating of mica pigments are known from U.S. Pat. No. 5,759,255.
WO98/13426 discloses coatings comprising water-based complex oligomeric silane systems.
When the above-mentioned hydrophilic organofunctional coupling reagents are used in aqueous systems, these reagents build up a water layer on the pigment surface. This results in blistering and swelling of the coating composition, thereby reducing the adhesion of the coating composition to the substrate.
A further disadvantage of aftercoating by means of hydrophilic reactive silanes is the promotion of osmosis which may adversely affect the coating composition.
It is a feature of the invention to overcome these disadvantages and to provide pigments which are advantageous relative to the prior art, being improved in particular in their stability in coatings and in the simultaneous improvement of further performance properties, such as reduced blistering, wetting and adhesion, and in the swelling behavior.
This feature can be achieved in accordance with the invention by the provision of a pearl luster pigment, treated with hydrophobic coupling reagents, in which no chemical bond is formed between the polymer and the organofunctional moiety. While not wishing to be bound by theory, it is believed that this result is obtained because the coupling agents, such as alkyl and aryl silanes, generally have no reactive groups. As a result, the coupling agents do not react with the polymer (binder) of the lacquer. As an example, the hydrolysed silane reacts with the hydroxyl groups of the metal oxides. The result can be the removal of water and the formation of an oxygen bridge between the metal oxide and coupling agent.
Specifically, aftercoated pearl luster pigments are provided on the basis of a platelet-shaped substrate coated with metal oxides and a top layer which is located on the metal oxide layer and possesses a multilayer structure, comprising a first layer, containing a hydroxide or oxide hydrate of the elements aluminum or silicon, a second layer, containing at least one hydroxide or oxide hydrate of the elements aluminum, silicon, cerium or zirconium, with the exception of the hydroxide or oxide hydrate which forms the first layer, and a third layer, containing at least one organic hydrophobic coupling reagent.
The first and second layer together are 1-3% by weight with respect to the base pigment, i.e., a platelet-shaped substrate coated with at least one metal oxide. In addition, the first and second layer may, in combination, have a thickness of 10-50 nm. Preferably, the first layer is aluminum oxide hydrate and the second layer is silicon oxide hydrate.
The pearl luster pigments prior to aftercoating (substrates) are pigments which comprise a platelet-shaped material, for example mica, kaolin or glass, and one or more metal oxide layers deposited thereon. The metal oxide layer may comprise, for example, titanium dioxide, titanium dioxide mixed with iron (III) oxide, iron (III) oxide, chromium oxide, zirconium dioxide, tin dioxide or zinc oxide. Pigments of this kind are available commercially under the designation Iriodin(copyright) (manufacturer: E. Merck, Darmstadt, Germany).
The coupling reagent is preferably selected from the group of the silanes, these compounds carrying at least one functional group selected from straight-chain or branched alkyl groups having 3 to 18 carbon atoms, unsubstituted or substituted by fluorine, and aryl groups unsubstituted or substituted by C1-C10 alkyl groups and/or nitro groups.
Among the above-mentioned coupling reagents, particular preference is given to the aryl-, alkyl- and fluoroalkyl-substituted di- and trimethoxysilanes. These include, for example, ethoxyphenyltrimethoxysilane, propyltrimethoxysilane, butyltrimethoxysilane, isobutyltrimethoxysilane, hexyltrimethoxysilane, octyltrimethoxysilane, 1H, 1H, 2H, 2H-perfluorodecyltrimethoxysilane and (3,3,3-trifluoropropyl)methyldimethoxysilane.
The coupling agent may be 0.2-1.2% by weight relative to the base payment, the platelet-shaped substrate coated with at least one metal oxide.
A further object can be to provide a process for obtaining the pearl luster pigments of the invention.
This feature can be achieved by the provision of a multistage process in which the substrate coated with metal oxides is suspended in water, heated at from 30 to 100xc2x0 C., preferably from 40 to 75xc2x0 C., the suspension is adjusted to a pH of from 3 to 9, preferably from 6 to 7, in a first stage one or more water-soluble metal salts are added, these salts are deposited in whole or in part as metal hydroxides and/or metal oxide hydrates, in a second stage by adding at least one water-soluble silicate, aluminum salt, cerium salt and/or zirconium salt at a pH in the range from 3 to 9, preferably from 6 to 7, the corresponding hydroxides or oxide hydrates thereof are deposited, and in a third stage at least one organic hydrophobic coupling agent is added and binds to the deposited oxide hydrate layer at a pH of from 3 to 9, preferably from 6 to 8. Subsequently, the pigment obtained in this way may be separated by sedimentation, washing and filtration and dried at from 80 to 160xc2x0 C., preferably from 120 to 160xc2x0 C.
If required, a sedimentation of the pigment may be carried out as a purifying step between the second and third stages.
The metal hydroxides and/or metal oxide hydrates employed in the process of the invention are hydroxides and/or oxide hydrates of aluminum, silicon, zirconium and cerium, preferably of aluminum and silicon.
Coupling agents used are preferably those mentioned above.
The pearl luster pigments coated in accordance with the invention are used in particular in the pigmenting of inks, such as printing inks, and plastics, such as polymer films, and coating compositions, such as paints, for example. They are also employed, however, in other areas where the above-mentioned improved properties play a part, such as the pigmenting of cosmetics, for example.
Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The following preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever.
In the foregoing and in the following examples, all temperatures are set forth uncorrected in degrees Celsius; and, unless otherwise indicated, all parts and percentages are by weight.
The entire disclosure of all cited applications, patents and publications, and corresponding DE application No. 10054981.0, filed Nov. 6, 2000 is hereby incorporated by reference.